We conducted an intercalibration program among eleven laboratories making measurements of N2O in the atmosphere in December 1976. The laboratories were in good agreement on the atmosperic concentration of nitrous oxide, with one exception. Excluding the one result which showed an anomalously low value, the other ten laboratories show a mean concentration of 323.5¿8.7 ppb v/v N2O for the Northern Hemisphere tropospheric background level for mid-1976. These results are in agreement with most recent atmospheric measurements of N2O, although some groups have reported values approximately 10% lower. However, the preponderance of recent evidence suggests very strongly that the concentration of nitrous oxide in the troposphere is between 315 and 330 ppb v/v.